It is a fact that there are people who are more resistant to cold than others: there are those for whom a simple breeze makes their skin crawl and it doesn’t happen without having a sweater at hand; and who even after the storm Filomena threw herself in a swimsuit into the snow. But the reasons behind the divergence between the ‘chilly’ and the ‘hot’ remain unclear. Now, researchers at the Karolinska Institutet in Sweden may have found the reason, a gene mutation and a lack of the protein α-actinin-3. The results have just been published in the magazine The American Journal of Human Genetics.

Skeletal muscles include fibers of two types, fast-twitch (white), which allow great efforts in a very short time, but which tire quickly; and those of slow contraction (red), which better withstand less demanding activities, but continued over time since they are more resistant to fatigue. Researchers found that the protein α-actinin-3, found only in fast-twitch fibers, is absent in nearly 20% of today’s humans – nearly 1.5 billion people – due to a mutation in the gene that encodes it. And the lack of this protein makes them more resistant to cold.

“The study suggests that people lacking α-actinin-3 are better at maintaining body heat and, in terms of energy, at withstanding harsher weather, but there has been no direct experimental evidence of this before,” he says Hakan Westerblad, professor of muscle physiology cell technology in the Department of Physiology and Pharmacology at the Karolinska Institute and lead author of the study. “Now we can show that the loss of this protein gives greater resistance to cold and we have also found a possible mechanism for this. For the study, 42 men between the ages of 18 and 40 were asked to sit in cold water (14 ° C) until their body temperature dropped to 35.5 ° C. During cold water immersion, the researchers measured the muscle electrical activity with electromyography (EMG) and muscle biopsies were taken to study protein content and fiber type composition.

The results showed that the muscle of people lacking α-actinin-3 contains a higher proportion of slow-twitch fibers and that, by cooling, they were able to maintain their body temperature more efficiently. Thus, instead of activating the fast-twitch fibers, which causes a shudder (or the well-known chills, which help keep you warm by making the muscle work), they increased the activation of the slow-twitch fibers that produce heat by increasing the contraction (muscle tone) of the baseline. That is, they seemed to conserve heat not by shivering, but by tensing the muscles, temporarily increasing muscle tone.

The evolutionary advantage that can become inconvenient. The mutation probably gave an evolutionary advantage during the migration to a colder climate, but in today’s modern society this energy-saving ability could increase the risk of diseases of affluence – such as obesity and hypertension – which is somewhat what we now want to direct our attention to, ”says Westerblad. Specifically, this strategy proved to be helpful when humans migrated from Africa to other places with colder climates. The team also investigated how a lack of α-actinin-3 affects the body’s response to physical exercise. “People lacking α-actinin-3 are less successful in sports that require strength and explosiveness, while there has been a trend toward greater capacity in these people in endurance sports,” he explains.